Joseph Krull, an executive at Flanders, N.J.-based Virtual Corporation, had a doctor stick an RFID tag from VeriChip under his skin on Jan. 10. The residual blemish amounts to a small red dot.

"It felt like a bee sting," he said during a meeting at the RSA Security Conference in San Francisco. "They use a syringe and a local anesthetic."

Privacy advocates have asserted that private information from the radio tags could be intercepted by corporations and identity thieves. Fears that governments could exploit the chips for tagging a person's whereabouts have alarmed libertarians and those with strong religious beliefs. On the other hand, tagging prisoners in some institutions has cut down on inmate violence.

Krull, for his part, got the chip for medical purposes. He's allergic to two medicines and has a small metal plate below his left eye that had to be put in following a skiing accident. By sweeping a reader over the chip, a doctor in an emergency could get a 16-digit password, which in turn would let the doctor access a Web site that would give the hospital Krull's name, regular doctor, emergency contact and other information.